Spin Glass AgMn Research Articles

Spin current study of spin glass AgMn using spin pumping effect

The role of spin glass as a spin current absorber was investigated by spin pumping effect in Ni81Fe19/Cu/spin glass Ag90Mn10 trilayer structure. The spin current was pumped to the spin glass layer via the Cu layer by means of the ferromagnetic resonance of the Ni81Fe19 layer. The damping constant of the ferromagnetic layer reflecting the intensity of the spin current absorption was evaluated by fitting the spectra with Lorentzian. At low temperatures, the spin current absorption in the spin glass layer arises sharply.

Quantum spin glass relaxation in UCu4Ni

Strong non-exponential relaxation due to magnetic disorder in UCu4Ni is observed in zero- and longitudinal-field μSR measurements in this material. Zero-field experiments reveal no evidence for spin freezing down to 20 mK. Qualitatively the shape of the muon asymmetry curves in longitudinal fields is strongly affected as the field is increased from zero and less so as the field is increased towards kOe values. Analysis of the time/field scaling (t/Hγ) for the asymmetry data, performed without reference to any specific decay function or model, together with the absence of spin freezing enables one to classify this system as a quantum-spin-glass; i.e., similar to UCu4Pd. However, in contrast with the behavior of the Pd material and similarly to the more conventional spin glass AgMn, UCu4Ni follows (t/Hγ) scaling only above applied fields of order 100 Oe. Unlike UCu4Pd which has been discussed extensively as partially ordered, disorder ought to be randomly distributed in UCu4Ni.

Experimental first order reversal curves (FORC) on Ising and Heisenberg spin glasses

The first order reversal curve (FORC) method is employed to analyse the low temperature hysteresis behaviour of two model 3d spin glasses: the Ising system Fe 0.5 Mn 0.5 TiO 3 and the near isotropic Heisenberg like system Ag(11 at% Mn). The hysteresis loops of the two samples are strikingly different (in accord with results from earlier investigations on spin glasses with different anisotropy). The derived FORC distribution of the Ising system is wide, centred around 0-bias field and in parts similar to what has been observed for interacting magnetic nanoparticle systems with wide particle size distributions. The FORC distribution for the Ag(Mn) spin glass on the other hand shows a sharp peak at a weak negative bias field and a coercive field of about - 80 and 200 Oe, respectively.

Magnetic relaxation of a perturbed equilibrium spin glass state

Magnetic relaxation experiments on a Ag(Mn) spin glass that has been subjected to temperature perturbations are reported. A dynamic quasiequilibrium state is created by allowing the spin glass to equilibrate for an extended time (100 000 s) at constant temperature in the spin glass phase. The “quasiequilibrium” (reached after 100 000 s) state is perturbed by a negative temperature cycle of different duration. The magnetic relaxation rate after the perturbation exhibits an enhanced relaxation rate at short observation times followed by a dip that extends well below the equilibrium relaxation rate at longer observation times.

Slow spin dynamics in non-fermi-liquid UCu5−xPdx, x=1.0 and 1.5

Low-temperature muon spin-lattice relaxation measurements in the non-Fermi-liquid heavy-fermion alloys UCu5−xPdx, x=1.0 and 1.5, indicate inhomogeneously distributed f-electron spin fluctuation rates, and exhibit a time-field scaling of the muon relaxation function indicative of long-lived spin correlations. In UCu4Pd the scaling exponent γ is small and temperature independent. In UCu3.5Pd1.5γ varies with temperature, increasing with decreasing temperature similar to spin-glass AgMn. This suggests that the spin-glass state found for x≳2 in UCu5−xPdx modifies the low-frequency spin dynamics in UCu3.5Pd1.5.

Probing exotic spin correlations by muon spin depolarization measurements with applications to spin glass dynamics

We develop a new method to probe the local spin dynamic autocorrelation function, using magnetic field dependent muon depolarization measurements. We apply this method to muSR experiments in the dilute Heisenberg spin glass AgMn(p at. %) at T>T_g where the correlations of the Mn local magnetic moment are strongly non-exponential. Our results clearly indicate that the dynamics of this spin glass cannot be described by a distribution of correlation times. Therefore, we analyze the data assuming a local spin correlation function which is the product of a power law times a cutoff function. The concentration and temperature dependence of the parameters of this function are determined. Our major conclusion is that in the temperature region close to Tg the correlation function is dominated by an algebraic relaxation term.

Memory and superposition in a spin glass

Non-equilibrium dynamics in a Ag(Mn) spin glass are investigated by measurements of the temperature dependence of the remanent magnetisation. Using specific cooling protocols before recording the thermo- or isothermal remanent magnetisations on re-heating, it is found that the measured curves effectively disclose non-equilibrium spin glass characteristics such as ageing and memory phenomena as well as an extended validity of the superposition principle for the relaxation. The usefulness of this "simple" dc-method is discussed, as well as its applicability to other disordered magnetic systems.

Magnetic field–time-scaling relations and exotic spin correlations: a μSR study of spin glasses

In this work we analyze a longitudinal field [H] μSR experiment in the spin glass AgMn(0.5 at%) using a cut-off power-law dynamical field auto-correlation function q(t)=2ct −x exp [−(νt) y] . This function allows us to account for the data for more than 2.5 orders of magnitude in magnetic field and two orders in time. Our results suggest that spin glass freezing is associated with a dynamical transition from a cut-off power law to a pure cut-off.

Probing the Spin-Spin Dynamical Autocorrelation Function in a Spin Glass above Tg via Muon Spin Relaxation.

We report zero and longitudinal field muon spin relaxation measurements in the metallic spin glass AgMn(0.5 at. %) at $Tg{T}_{g}$. We find that muon polarization obeys a time-field scaling relation which allows us to distinguish between three possible forms of the spin-spin dynamical autocorrelation function: a power law, a stretched exponential, and a cutoff power law. We also discuss the evolution of the muon relaxation line shape as the temperature approaches ${T}_{g}$.

Atomic and magnetic short-range order in Ag-Mn spin-glass alloys.

To understand the relationship between the magnetic properties and local arrangements of Mn spins in Ag-Mn spin-glass alloys, we have measured the magnetic susceptibility, and x-ray and diffuse-neutron-scattering intensities for three single crystals. The x-ray results show that the Mn atoms are found to have a preference for linking up along 〈100〉 directions. This tendency increases with increasing Mn content up to around 20.8 at. % Mn. However, this tendency decreases abruptly at 28.1 at. % Mn. The neutron results for the spin-glass state of Ag-20.8 at. % Mn alloy show that the nearest spin pairs have strong antiferromagnetic coupling and the second-nearest spin pairs along the 〈100〉 directions are correlated ferromagnetically with each other. These ferromagnetic spin chains are found to develop inside the atomic short-range-ordered region of Mn atoms. In addition to the existence of these spin chains, we have also observed longer-range spin correlation with about twice the correlation length of the atomic one. The amplitude of the magnetic susceptibility at the freezing temperature and the paramagnetic Curie temperature are found to have a strong relationship with the short-range spin structure. We believe that the longer-range spin correlations cannot grow without conflicting with the short-range spin structures inside the atomic short-range-ordered region of Mn atoms. This may become a key point to understand the spin-glass freezing mechanism.

Atomic and Magnetic Short-Range Order in Ag-Mn Spin-glass Alloys

Atomic and Magnetic Short-Range Order in Ag-Mn Spin-glass Alloys

“Universality” of finite size effects in CuMn and AgMn spin-glasses

Recent measurements of the quasi-static spin-freezing temperature, Tf, of thin Cu1-xMnx layers (x = 0.04, 0.07, 0.14) in magnetron sputtered multilayer samples (MS) of CuMn alternated with Cu or Si revealed two different apparently “universal” curves for the ratio TfTfb is the bulk freezing temperature for a given x) for CuMnCu and CuMnSi MS. We show that cooling the substrates leaves the CuMnCu MS data unchanged, but moves the nSi MS data closer to the “universal” curve for the CuMnCu MS show also that TfTfD for AgMnAg MS agrees with the CuMnCu MS “Universal” curve.

“Universality” of finite size effects in cumn and agmn spin-glasses

Recent measurements of the quasi-static spin-freezing temperature, T f, of thin Cu 1-xMn x layers (x = 0.04, 0.07, 0.14) in magnetron sputtered multilayer samples (MS) of CuMn alternated with Cu or Si revealed two different apparently “universal” curves for the ratio T f T f b is the bulk freezing temperature for a given x) for CuMn Cu and CuMn Si MS. We show that cooling the substrates leaves the CuMn Cu MS data unchanged, but moves the n Si MS data closer to the “universal” curve for the CuMn Cu MS show also that T f T f D for AgMn Ag MS agrees with the CuMn Cu MS “Universal” curve.

Dynamics of a spin glass along the hysteresis cycle

A correlation is observed between the ESR spectra (at 1.12 GHz) and the magnetization inversions of the spin glass AgMn for a Mn concentration of 8.3 at. %. This is because of rearrangement of the spin system during the magnetization inversions. The widths of the ESR lines W↑ und W↓ which correlate, respectively, with the reversals to positive and negative magnetization are different. This is ascribed to different values of the effective anisotropy constant K during the two reversals. Thus a relationship is obtained between the ESR linewidth and the anisotropy constant.

Anomalous behavior of electron-spin resonance on a spin glass at Tg

We report measurements on Ag Mn spin glass (10.3 at% Mn) for 0.1 T g < T < 1.6 T g at multiple frequencies between 0.6 and 9.5 GHz. For T < T g, the isotherms in a diagram of frequency ω/2π vs. resonant field intersect the ω-axis at ω ≠ 0, like in antiferromagnets. This indicates the existence of an anisotropy energy (density) or constant K. The isotherms above T g, however, meet the origin, denoting that K(H = 0) = 0. An anisotropy constant K( H ≠ 0) ≠ 0 above T g is attributed to the critical fluctuations suppressed by the static applied field. The anomalous behavior of the linewidth at T g is also discussed qualitatively.

Study of temperature dependence in the electron-spin resonance on spin glass [formula omitted]Mn below T g

The ESR of Ag Mn spin glass has been investigated below T g. The field-cooled shift and width increase with decreasing temperature, and go through a maximum. In contrast, no clear maxima are observed in a zero-field-cooled experiment. The field-cooled and zero-field-cooled data are analysed in the frame of the hydrodynamic theory. The existence of a maximum in the field-cooled shift is due to a temperature dependent competition between the remanent magnetization and the anisotropy constant K. Since in the zero-field-cooled experiment the remanence is very low, there is no temperature dependent competitor for K, and consequently no maximum for the shift.

Resistivity of noble-metal-transition-metal spin glasses

Following the existence of a 'hole' in the local field distribution P(H), the authors have divided the spin system of a spin glass into two groups: those lying at the lower bounds of P(H) are 'effective free' spins and the rest are interacting spins. New expressions for 'effective free' spin populations and interacting spin populations are incorporated into a modified Kondo theory and a spin-diffusion theory respectively. They have thereby obtained a model for magnetic resistivity. The authors analysed experimental data on AgMn, AuMn and CuMn spin glasses, using this model for best fitting of the data. The model gives a satisfactory description of the experimental behaviour for all these alloys. A new resistivity maximum is observed in the interacting spin resistivity at a temperature TSG, close to Tf, the temperature for the AC susceptibility cusp. The temperature Tm, of the resistivity maximum depends on impurity spin S, and it is related to the lower bounds Hc, of P(H).

The low-field Hall effect and susceptibility of archetypal spin glasses-evidence for a second magnetic transition

The authors have measured the very-low-field Hall coefficient and AC susceptibility of a number of Au-Mn, Au-Fe and Ag-Mn spin-glass alloys over the temperature range 4.2-300 K. The relation R11=R0+(Rs/ mu 0) chi m has been found to be inapplicable in all the alloys, but the modified relation RH=R0+(Rs/ mu 0) chi M0.75 fits all the alloys over the whole temperature range investigated. Changes of R0 and Rs occur across the freezing temperature and also at a temperature in the paramagnetic region which coincides with the temperature where deviations from Curie-Weiss behaviour take place. It is suggested that at this temperature, a subtle second magnetic transition occurs where individual moments first couple under the RKKY interaction. As a result, spin glasses are characterised by three regions; spin glass, intermediate paramagnetic and Curie-Weiss paramagnetic regions. The upper transition temperature appears to be closely associated with the temperature at which the magnetic component of the electrical resistivity reaches a maximum in Au-Mn alloys. The changes of R0 and Rs across the freezing temperature have been associated with changes in the anisotropy of ordinary and skew scattering resulting from the combination and blocking of clusters immediately above the freezing temperature.

Field dependent critical fluctuations aboveT g in the ESR line width of the spin glassAgMn

The ESR of the spin glassAgMn (2.7 and 9.6 at %) has been investigated below and aboveT g(0.1T g<T<5T g) at various microwave frequencies. The analysis yields:1) No explicit frequency dependence but strong magnetic field effects, inherent with ESR-experiments.2) Part of the excess line width is identified as critical spin fluctuations, following a power law. However, because of the presence of the applied field, the reduced temperaturet is not a good scaling variable. We choose the non-linear susceptibility X s divided byH 2, which scales as the order parameter susceptibility. The experiment yieldsW ex∞(X s /H 2) p ,p=0.42. From this we deduce νz≈3.

High-sensitivity broadband microwave spectroscopy with small nonresonant coils

The use of a small, nonresonant, coil of μm dimensions as the microwave magnetic field structure of a broadband electron-spin-resonance (ESR) spectrometer is described. The coil is driven by a broadband microwave generator which operates between 0.1 and 8.5 GHz. The samples may fill the coil to approximately 100%. It is shown that for small size samples this system offers higher sensitivity than a conventional cavity spectrometer. Because the system is broadband, either frequency scans or the conventional magnetic field scans can be used to traverse the resonance. Examples for DPPH and for the spin glass AgMn using this method are reported. Since the sample coil is small, it has many potential applications, such as insertion into the mixing chamber of a dilution refrigerator or high-pressure cell, efficient use of power in ENDOR and other double resonance experiments, and rapid recovery from transients in pulsed ESR experiments.